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The mechanism of the spontaneous intercalation of Li metal into graphite electrodes is highly relevant for aging mechanisms and pre-lithiation of Li-ion cells. In the present work, we introduce a method to investigate this mechanism via measuring the opencircuit-potential (OCP). Experiments without electrolyte, with organic solutions without and with LiPF 6 reveal details on the reaction mechanism at 29 °C. The electrodes are investigated by Raman spectroscopy and glow-discharge optical emission spectroscopy (GD-OES) depth profiling to reveal the spatial distribution of the lithiated phases. The analytical information is enriched by simulations with the Battery and Electrochemistry Simulation Tool (BEST). The combination of tools gives interesting insights into the behavior of negative electrodes regarding re-intercalation of deposited Li into graphite and its kinetics, development of inhomogeneities during aging, as well as pre-lithiation and post-mortem analysis methodology.

A new in situ optical microscopy set-up is introduced which allows direct observation of cross-sections of Li-ion full cells in combination with simultaneous recording of electrochemical data. The method is validated by comparison of electrochemical data from coin full cells. Color changes give insights into processes on the electrode and particle level, such as lithiation behavior and electrode thickness changes. Our observations allow the evaluation of (i) the speed of lithiation fronts for LiC12 and LiC6 through anode coatings, (ii) estimation of apparent diffusion coefficients from analysis of the color distribution in single graphite particles, as well as (iii) electrical de-contacting and re-contacting of single graphite particles in connection with (iv) electrode thickness changes. Furthermore, our direct observations from the inside of full cells give indirect insights into aging phenomena such as Li plating and SEI growth.

In Lithium-ion batteries, the graphite anode is known to undergo a noticeable chromatic change during lithiation and de-lithiation by forming graphite intercalation compounds. Additionally, the graphite anode primarily contributes to the volume change of the battery. Using a novel in-situ optical microscopy setup for imaging cross-sections of Li-ion full cells, both effects can be studied simultaneously during charging and discharging. In this work, we describe feature extraction methods to quantify these effects in the image data (3730 images in total) captured during the lithiation and de-lithiation process. Automated and manual evaluations are compared. The images show graphite anodes and NMC 622 cathodes. For colorfulness, we evaluate different methods based on classical image processing. The metrics calculated with these approaches are compared to the results of ColorNet, which is a trainable colorfulness estimator based on deep convolutional neural networks. We propose a supervised semantic segmentation approach using U-Net for the layer thickness measurement and the anode dilation derived from it.

In Li-Ion batteries, the graphite anode undergoes chromatic changes from grey (unlithiated graphite) to blue (LiC18) to red (LiC12) to gold (LiC6) during lithiation.1 When charged with high current rates, lithiation gradients are observable due to mass transport limitations within the anode.2 Due to high lithiation degrees at the surface, the anode potential decreases locally and the possibility of Li deposition is increased.3 We introduce a new in situ optical microscopy set-up, which allows a direct observation of the cross-section of Li-ion full cells in combination with simultaneous recording of electrochemical data (Figure 1). Extensive analysis of the chromatic changes from the graphite electrode surface to the current collector gives insights into the lithiation processes on electrode and particle level. The set-up was validated by comparing the electrochemical results with data from coin full cells. The propagation of lithiation fronts for LiC12 (~ 3300 µm2 min-1) and LiC6 (~1260 µm2 min-1) at 1C through the graphite electrode coating were determined as well as the estimation of apparent solid-state diffusion coefficients in the order of 10-10 cm2 s-1 from analyzing the phase propagation within single particles at C/10. Additionally, the expansion of all components of the whole cell can be observed individually. We found that the graphite contributes mainly to the cell expansion, both irreversibly (4%) and reversibly (4-13%). Directly observing the described phenomena in the full cell can give insights into aging mechanisms of the materials. Figure 1 Exemplary image from the video at the end of charge of an in situ measurement (cycling at C/10) of a graphite-NMC 622 full cell cross-sections. References P. Maire, A. Evans, H. Kaiser, W. Scheifele and P. Novák, J. Power Sources, 155(11), A862 (2008). M. Weiss, R. Ruess, J. Kasnatscheew, Y. Levartovsky, N. R. Levy, P. Minnmann, L. Stolz, T. Waldmann, M. Wohlfahrt-Mehrens, D. Aurbach, M. Winter, Y. Ein-Eli and J. Janek, Adv. Energy Mater., 11(n/a), 2101126 (2021). T. Gao, Y. Han, D. Fraggedakis, S. Das, T. Zhou, C.-N. Yeh, S. Xu, W. C. Chueh, J. Li and M. Z. Bazant, Joule (2021). Figure 1